1. Field of the Invention
This patent specification relates to apparatuses and methods for wettability characterization of solid material. More particularly, the patent specification relates to apparatuses and methods for characterizing wettability of solid material by positioning the material at the interface between two fluids and applying an external force, such as with a centrifuge.
2. Description of Related Art
Wettability is one of the major parameters, which can be used to characterize oil recovery efficiency. If a porous rock is oil wet, which is the case when the contact angle of an oil/water system on the surface of the solid approaches 180°, then only a small fraction of oil is recoverable. This can be observed, for example, in carbonate reservoirs. In a case of water wet rock, which is the case when the contact angle of an oil/water system on the surface of the solid approaches 0°, the oil recovery efficiency is much higher. Contact angle ranges have been introduced to characterize the water-wet, intermediate wet, and oil-wet characteristics of reservoir rocks. See, L. E. Treiber, D. L. Archer and W. W. Owens, Laboratory Evaluation of the Wettability of Fifty Oil-Producing Reservoirs, Soc Pet Eng J 12, 531 (1972). It is important to evaluate the contact angle to characterize oil recoverability. Contact angle is most often measured directly by optical means although alternative non-optical techniques are also described in the art. The optical contact angle measuring technique has the following inherent problems: (i) a geometrically and energetically homogenous macroscopic solid surface is needed; (ii) the advancing and retreating contact angles should be measured independently and the equilibrium contact angle needs to be calculated based on some theoretical approach; (iii) the contact angle measurements need some equilibration time, hence these measurements are time consuming; (iv) sophisticated image capturing and analyzing software is necessary; and (v) one of the phases should be optically clear. One also has to realize that only one additive with a specified concentration could be tested with the same instrument at a time because of the equilibration time and complexity of measurements. Consequently it takes a long time to screen the effect of added chemicals as a function of chemistry and concentration on the contact angle. Therefore it would be beneficial to develop a screening methodology, which would request less sophisticated experimental apparatus and would make possible parallel screening of a large number of samples.
Currently in the oilfield industry, the wettability is preferably characterized by the US Bureau of Mines (USBM) Wettability Index based on the work done to introduce each phase into a core at residual saturation for that phase. Starting with a core saturated with oil at irreducible water saturation, water is allowed to imbibe spontaneously into the core followed by forced imbibition of water until residual oil saturation is attained. Next oil is allowed to imbibe spontaneously into the core followed by forced imbibition of oil till the core reverts to irreducible water saturation. Throughout the process the capillary pressure is plotted versus saturation. The area under the curve between the capillary pressure line and the x-axis represents the work done to force the given phase into the core. The USBM Wettability Index is the difference of the logarithm of the area above the Pc=0 axis and the area below the Pc=0 axis. The centrifugation of a macroscopic core sample could be used to establish and calculate the equilibrium capillary pressures. The details of this procedure were published in: D. Tiab and E. C. Dionaldson, Petrophysics, Gulf Publishing Co., (2004).
The wettability index characterizes wettability but does not provide the quantitative information about a wetting system. The Amott wettability index, IA, is also used to characterize wettability of reservoir rocks. Sett, E. Amott, Observations relating to the wettability of porous rock, Trans. AIME 216, 156 (1959). It is based on spontaneous imbibition and forced displacement of water and oil. It is defined with the following formula:
      I    A    =                    V        OSP                    V        OT              -                  V        WSP                    V        WT            
Where VWSP is the volume of water displaced by spontaneous imbibition of oil; VWT is the total volume of water displaced by the forced imbibition of oil; VOSP is the volume of oil displaced by spontaneous imbibition of water; VOT is the total volume of oil displaced by the forced imbibition of water.
Applying an external force from a centrifuge to a particle at an air-water interface has been investigated. See, C. W. Nutt, Froth Flotation: The adhesion of solid particles to flat interfaces and bubbles, Chem. Engineering. Sci. Vol. 12, pp 133 to 141 (1960), discusses an investigation of experiments and derived equations for the force and work required to detach a particle from an air-water interface as a function of solid and liquid densities, the surface tension and the angle of contact. However, in this paper only air-water systems were investigated so it was possible to neglect the density of air, which led to simplified mathematical formulations. Also, the curvature of the meniscus was described by only single curvature, and therefore only one curvature was used in the Laplace Equation. The rotation speed was calculated and then compared with their experimental results. Finally, there is no discussion of attempting to determine wettability.